That will be extremely bright.
I replaced 60W 240V candle lamps with 35W 12V in a central
room pendant, and the light output is very noticably brighter,
probably double, but unfortunatelty I didn't measure before.
I still run them in parallel at 12V, although I have rewired
to cope with higher current draw (and high frequency operation
from electronic transformer which requires much thicker/stranded
wire due to skin effect).
You should probably be looking at using 10W, or 20W max.
I'm not sure what additional failure modes you introduce
by having the potential for 240V across a failing 12V
capsule lamp, but I would not rule out the possibility
of a low pressure capsule exploding at switch-on.
Thermistor will help here though. Finding the dead
capsule will be painful. See Big Clive's chandelier
http://www.bigclive.com/halogen.htm
for an idea using parallel neon indicator lamps.
Lamp bases for 12V capsule lamps are not necessarily
designed to be safe when operating at 240V WRT ground.
See:
http://www.cucumber.demon.co.uk/lights/adapter.jpg
These are designed to position the filaments in the
same (optimal) position as they were with the mains
lamps they replace.